I have enabled the multi-tenancy feature for my firebase project in the GCP console following this guide but now there's no way to disable it back.
Will this affect any of project's behaviour?
As per the Public Documentation, currently there is nothing mentioned about how to disable multi-tenancy , however Google Support is aware of the issue and working on it. There will be some update in near future but currently it's not possible in UI or SDKs.
Will this affect any of the project's behaviour?
As per Firebase Documentation , multi-tenancy is not recommended for the Firebase Projects as mentioned with a reason :
Multi-tenancy can lead to serious configuration and data privacy concerns, including unintended issues with analytics aggregation, shared authentication, overly-complex database structures, and difficulties with security rules.
Generally, if a set of apps don't share the same data and configurations, strongly consider registering each app with a different Firebase project.
For example, if you develop a white-label application, each independently labeled app should have its own Firebase project, and the iOS and Android versions of that label should be in the same Firebase project. Each independently labeled app shouldn't (for privacy reasons) share data with the others.
To get more information related to the Firebase Authentication, you can refer to the doc.
If you want to continue in a multi tenant project below are some points which may help you perform certain activities on a multi tenant.
There are situations, however, where you want users to be manually or programmatically created by an administrator, either using the Admin SDK or Firebase console. In these cases, you can disable user actions from the Firebase Authentication Settings page, which prevents account creation and deletion by end-users. If you are using multi-tenancy, you will need to make an HTTP request to disable these features on a per-tenant basis.
You can also Manage Identity Platform tenants programmatically by User management,Identity verification,Import users,Access control with custom claims,User session management,Email action links,Tenant management by Create, list, get, update, delete tenants for a specific Identity Platform project , Manage OIDC and SAML providers on tenants.